1. Field of Invention
This invention relates to reusable electric paper and, more specifically, reusable electric paper that discourages tampering by providing evidence of tampering.
2. Description of Related Art
FIG. 1 shows one form of electric paper 1 which consists of a polymer substrate with little balls 20 embedded that are one color, for example, white 30, on one side and another color, for example, black 40, on the other. Such electric paper is described in U.S. Pat. No. 5,604,027, incorporated herein by reference. Under the influence of an electric field each ball rotates so that either one colored side or the other is on top and therefore visible to a viewer viewing the electric paper from the top.
Printing on electric paper is accomplished by imposing an electrical pattern over the sheet the electrical pattern being created by a voltage difference between the top side of the sheet and the bottom side of the sheet. A typical way to do this is to pass the sheet under a charging bar. As the sheet passes under the bar, voltages are applied along a set of closely-spaced electrical contacts one for each pixel or ball.
While one form of electric paper is described above, many forms of electric paper are known such as electric paper including other types of rotating elements, like cylinders, or electrophoretic or liquid crystal forms of electric paper.
Audit trail documents are found throughout our society. For example, most items shipped from a factory to a customer typically include a document on the outside of the packaging to collect the signatures from the various people who handle the items. These documents often have multiple sheets of regular paper with carbon paper separators so that each person can retain a record of their signature and the transaction history up to that point. In today""s world, computers are becoming more and more involved in transactions involving audit trails. For example, many shippers are now using computers to streamline their operations, including reducing the paperwork associated with their internal audit wails. A problem associated with such use of computers is that audit trail transactions often occur between people from different organizations. Although both organizations involved need a record of the on, one or the other organization may not be computerized or, even if both organizations are computerized, their computers may not be compatible with each other. Such incompatibility or lack of computerization results in transaction history becoming scattered among computer and paper records rather than being recorded on a single audit trail document.
These problems are addressed by the invention by providing a tamper-evident electric paper. One example of tamper-evident electric paper of the invention is made of two sheets of electric paper glued together after the top sheet has been erased to white and the bottom sheet has been printed with a uniform pattern. The pattern of the bottom sheet could be for example a grid of alternating black and white pixels. Writing on the tamper-evident electric paper would cause the addressed pixels to turn to, for example, black on both the top and bottom sheets. Erasing (e.g., restoring the pixels to white) a portion of the tamper-evident electric paper would not only restore the erased portion of the top sheet to white but would also change the corresponding portion of the bottom sheet to white thereby erasing not only the written image on the bottom sheet but also the uniform pattern on the bottom sheet. As a result, any erasing performed on the tamper-evident electric paper is evidenced by destruction of the uniform pattern on the bottom sheet. If the tamper-evident electric paper was subjected to the appropriate electric field required to restore the uniform pattern to the bottom sheet in order to try to hide the erasing, the uniform pattern would also be visible on the top sheet.
In one aspect of the invention a permanent glue is used to bond the top sheet to the bottom sheet, making the resulting tamper-evident electric paper virtually impossible to erase without detection. However, the tamper-evident electric paper can be used only once as tamper-evident electric paper unless the two sheets could be separated. It could, however, always be reused as regular electric paper.
The tamper-evident electric paper can be used for audit trails that may or may not involve computers. The paper can be signed by a pen that creates an electrical field between its tip and a uniform electrode on the other side of the electric paper sheet. When computers are involved in the audit trail, a jack-in-the-box display can be used by inserting the audit trail tamper-evident electric paper into the display. Signatures and-other entries are captured simultaneously into a computer attached to the jack-in-the-box display and onto the electric paper. Additionally, a scanning version of the display could allow the audit trail document to be stored into the computer and/or copied onto another sheet of electric paper to generate a record of the transaction that can be retained while the audit trail document continues to follow its trail.
The invention also provides a reusable tamper-evident electric paper that uses, for example, balls that require different electrical field strengths for rotation. The rotation of these balls follows a threshold-like behavior. Fields below a given value do not cause ball rotation, whereas fields above this value do. It is known that smaller balls commence rotation at lower electrical field strengths than do larger balls. Hence, the tamper-evident layer is made out of smaller balls that can be written at a lower field strength than the other layer. To reuse the tamper-evident electric paper, a new tamper-evident pattern is printed by using the higher voltage to erase everything and then using the lower voltage to print the tamper-evident pattern on the tamper-evident layer without changing the white of the other layer. While undetected tampering is possible with this type of tamper-evident electric paper, it would require a printer that generates both the higher voltage and the lower voltage.
The invention also provides a single sheet embodiment that has a background pattern printed on the sheet prior to use. The background pattern is a complex pattern such as, for example. encryption.